Abstract

The 1.875–1.86 Ga Great Bear Magmatic Zone is a linear belt, 100 km wide by 800 km long, of low TiO2, high Al2O3 volcanic rocks ranging continuously from basalt to rhyolite, cut by allied hornblende-biotite–bearing plutonic rocks. Comprehensive study of the facies, volcanic styles, chemistry, and petrography indicates that it is similar to Cenozoic continental arcs. Preservation of high-level volcanic rocks suggests that the zone never had an unusually thick crust and therefore is similar to some intra-arc depressions, such as the Mesozoic-Paleogene depression of Chile, common in younger continental arcs. These long, linear depressions are the sites for voluminous pyroclastic volcanism, contain sections tens of kilometres thick, and are zones of subsidence whose surface remains close to sea level. They originate when the mass of basaltic andesite arriving at the base of the crust equals the mass of vitric ash removed from the areas by high-level atmospheric transport during ash-flow eruptions.